1. Field of the Invention
The present invention relates to a continuous hot rolling process for making thin steel strip.
2. Description of the Prior Art
In recent years, hot rolled sheets have been used in place of cold rolled sheets from the viewpoint of cost savings and, particularly recently, there has been a demand for hot rolled sheet having good formability.
In general, to impart good formability to hot rolled sheets, it is necessary that the temperature of the hot strip immediately after finish rolling is completed (hereinafter referred to as "finisher delivery temperature") should be kept equal to or higher than the Ar.sub.3 transformation temperatures. However, in the case of thin strip having an extremely small (gauge) thickness (a thickness of typically 0.8 to 1.2 mm), it is difficult to realize a finisher delivery temperature sufficiently high to achieve the desired formability of such thin strip since a great temperature drop occurs during finish rolling and high rolling speeds cannot be obtained.
The reason why thin strip cannot be rolled at high rolling speeds is that the phenomenon of "waddling" occurs on a hot-run table, and, worst of all, it becomes impossible to continue rolling. "Waddling" is a phenomenon in which a strip cannot run smoothly in rolling. The reason for this is that the strip waves or is irregularly contracted in width or is partly bulged. The waddling phenomenon is well known in the art. For this reason, in a conventional rolling process, a hot strip is rolled at a low speed called threading speed such that no waddling occurs until the leading head of a hot strip reaches a coiler and, after the strip head has reached the coiler, the rolling speed is increased. However, in such a conventional rolling process, as the thickness of hot strip becomes smaller, it is necessary to correspondingly reduce the threading speed at which the head of a hot strip is rolled, with the result that the leading end portion of the hot strip is not overheated and at the same time a portion following the leading strip head is also cooled. It is therefore impossible to maintain a finisher delivery temperature necessary for imparting good formability to the aforesaid thin hot strip over a significant length thereof.
To solve the above-described problem, Japanese Patent Examined Publication No. 52-15254 proposes a rolling process comprising the steps of rolling the head of a hot strip at high speed, then reducing the rolling speed of a finishing mill during the period from the moment at which the head of the thus-rolled hot strip passes through the final finishing mill to the moment at which the strip head reaches a coiler and subsequently increasing the rolling speed. This is a method of utilizing the inertia of the strip head created by high speed rolling to eliminate waddling by speed reduction, but involves the disadvantage that, since strip temperature lowers during speed reduction, the finisher delivery temperature obtained when the strip head reaches the coiler becomes lower than a desired temperature. Therefore, according to the past level of techniques, it has been extremely difficult to maintain a finisher delivery temperature sufficiently high to impart desired formability to thin strip having an extremely small thickness.